Polarizing optical interferometer having a dual use optical element

ABSTRACT

A system for non-destructively measuring an object and controlling industrial processes in response to the measurement is disclosed in which an impulse laser generates a plurality of sound waves over timed increments in an object. A polarizing interferometer is used to measure surface movement of the object caused by the sound waves and sensed by phase shifts in the signal beam. A photon multiplier senses the phase shift and develops an electrical signal. A signal conditioning arrangement modifies the electrical signals to generate an average signal correlated to the sound waves which in turn is correlated to a physical or metallurgical property of the object, such as temperature, which property may then be used to control the process. External, random vibrations of the workpiece are utilized to develop discernible signals which can be sensed in the interferometer by only one photon multiplier. In addition the interferometer includes an arrangement for optimizing its sensitivity so that movement attributed to various waves can be detected in opaque objects. The interferometer also includes a mechanism for sensing objects with rough surfaces which produce speckle light patterns. Finally the interferometer per se, with the addition of a second photon multiplier is capable of accurately recording beam length distance differences with only one reading.

This invention was made with government support under contract no.DE-FC07-89ID12830 awarded by the Department of Energy. The governmenthas certain rights in this invention.

This is a division of application Ser. No. 785,787 filed Oct. 31, 1991,now U.S. Pat. No. 5,286,313.

This invention in general relates to a system, apparatus and method formeasuring wave motion in an object to control a process, and morespecifically to control industrial processes where heat is inputted ortaken away from the object.

The invention is particularly applicable to and will be described withspecific reference to measurement of workpiece temperature and otherphysical and chemical characteristics of the workpiece to controlindustrial processes applying heat to or from a workpiece. However theinvention has significantly broader applications and is not limited toheat processes. Further, the invention has additional unique featureswhich make it suitable for applications other than process control suchas its use as a polarizing interferometer, per se.

INCORPORATION BY REFERENCE

A report published by the National Technical Information Service, UnitedStates Department of Commerce, entitled "Development and Evaluation ofWorkpiece Temperature Analyzer for Industrial Furnaces", DOE/ID12830-1(DE91004352) was authored by several of the inventors and released forpublic distribution on or about Dec. 31, 1990, although dated May, 1990.The NTIS report in its entirety is incorporated by reference herein andmade a part and parcel hereof. In addition, applicants also incorporateby reference Gallagher et al U.S. Pat. No. 3,694,088 and page 61 of theApril, 1991 issue of NASA Tech Briefs as indicative of backgroundmaterial disclosing polarizing, phase-shift interferometers so thatdetails relating to phase-shifting interferometers need not be explainedin detail herein.

BACKGROUND A.) Industrial Heat Processes:

Industrial heat processes are controlled by measuring the temperature ofthe object or the workpiece which is either heated or cooled. Thetraditional method of determining workpiece temperature in an industrialfurnace is to measure the surface temperature of the workpiece with aradiation pyrometer, or a thermocouple positioned near the workpiece ora contact thermocouple. There are limitations with both devices. Contactthermocouples have limited use because they scratch the workpiecesurface when used in a continuous process i.e. strip lines, or theyrequire drilling of a hole in the workpiece if used in a batch furnaceor, in other applications they cannot be used. Radiation pyrometers aretraditionally used for non-contact surface temperature measurements butsuffer from inherent inaccuracies because of interferences by radiationfrom furnace hot walls and gases in the furnace. Further, the accuracyof radiation pyrometers is adversely affected by varying emissivity ofthe workpiece during thermal processing. The emissivity encountered incertain strip applications i.e. galvanizing, aluminizing, galvannealing,etc. preclude pyrometer direct applications in the sense that presentschemes utilize a pyrometer and one of two emissivities. If one iswrong, the other is assumed correct. In those instances when contactthermocouples or pyrometers cannot be employed, secondary measurementsare obtained and correlated to the expected temperature of the workpieceto control the process. For example, the temperature of the furnace gasis sampled and by means of empirical equations the process iscontrolled. In plasma arc applications or induction heating applicationsthe electrical power inputted to the workpiece is controlled. Inaddition other measurements are taken and the process is controlled bythe combination of measurements. For example, the furnace gas is sampledand its makeup analyzed to determine the extent that the heat treatingprocess has progressed. Again, all of these controls are secondary inthat something other than the workpiece is measured and from thatmeasurement an expected characteristic of the workpiece is extracted.

With respect to measurement of temperature, traditional, nondestructivetemperature measuring instruments lack any capability to measuretemperature gradients within the workpiece. There are thermal processeswhere temperature uniformity (plus or minus 5° F.) from surface to thecore of the workpiece is critical to achieve uniform phasetransformation reactions in the bulk of the workpiece. Traditionalprocessing techniques provide a predetermined hold or soak time at whichuniformity is to be achieved and this results in increased process cycletime etc.

With respect to measuring physical or chemical properties of theworkpiece, there are no instruments which can nondestructively, directlymeasure the properties of the workpiece during the heat treat process,although theoretically there are, of course, ways to actually measuretemperature gradients. As indicated above, secondary measurements areobtained and correlated to what the expected properties of the materialwould be. For example, in the heat treat carburizing process, thefurnace or process gas is sampled to determine its carbon content andbased on the measured variation in carbon content of the furnace gas theprocess is controlled on the underlying assumption that the carbondisassociated from the furnace atmosphere is uniformly infused into theworkpiece case.

B.) Ultrasound Waves:

As noted in the NTIS report a significant body of information has beenpublished on the generation of elastic waves in solids. It is known thatwhen transient changes in the structure of a solid occur, elastic wavesare generated on the surface and in the bulk of the workpiece. It isknown that there are four types of waves which can propagate in solids,namely longitudinal and/or shear, Rayleigh or surface and Lamb waves.The longitudinal and/or shear waves travel through the bulk of the solidwith the longitudinal waves being almost twice as fast as the shearwaves. The Rayleigh waves travel only on the surface of the solid withspeeds slightly less than the shear waves. The Lamb waves propagate onlythrough very thin plates and have been used to measure the thickness ofthese plates. Longitudinal bulk waves and shear bulk waves have alsobeen extensively used for detection of flaws, measurement of elasticproperties of solids and monitoring of steel solidification.

It is known to use lasers to generate ultrasound waves on the surface ofthe workpiece. See for example Kaule U.S. Pat. No. 4,144,767. Further,it is known in the literature search of the NTIS report that three typesof waves (longitudinal, shear, and Rayleigh) can be produced by laser inan unheated environment in aluminum, brass and various types of steelwithout any surface damage.

All of the references uncovered in the NTIS literature search usedtransducers i.e. conventional piezoelectric transducers to detect thesound wave. Clearly, placing the transducers in a heated environmenteither destroys the transducer or, at the very least requires extensivecorrecting circuitry to compensate for the temperature effect on thepiezoelectric device which in turn can adversely affect the readingsfrom the device.

C.) Optical Interferometers:

As discussed in the literature search of the NTIS report, the industrialuse of an optical interferometer to detect the movement of ultrasoundwaves at elevated temperature has not been uncovered. This is notsurprising when it is considered that normal applications forinterferometers require precise optical path lengths be established forthe reference and signal light beams which cannot exist in an industrialsetting. Accordingly, considering only variations in path beam lengtharising in industrial applications, one would not expect aninterferometer to have the sensitivity to consistently measureultrasound wave movement in an industrial setting with typicalinterferometer such as Twyman-Green, Michelson, Mach-Zehnder,Fabry-Perot. At the same time within the optical interferometer art,phase-shifting interferometers are well known and it is a characteristicof phase-shifting interferometers that the optical beam paths need onlybe set equal to one another within the coherence length of the beamlight. However, phase-shifting interferometers require severalphase-shifts in the reference beam to obtain the measurement i.e. seefor example Gallagher U.S. Pat. No. 3,694,088, and the time required togenerate multiple phase-shift readings, until the present invention,would prevent the use of phase-shifting interferometers to measure thesurface movement of an object in response to an ultrasonic wave inducedtherein.

SUMMARY OF THE INVENTION

Accordingly, it is one of the principle objects of the invention todirectly and nondestructively measure a property of a solid object toachieve improved control of industrial processes, typically processeswhich are used in the manufacture of the object.

In accordance with a principle feature of the present invention acontrol system, apparatus and/or method is provided for regulatingindustrial processes by nondestructively sensing surface wave movementon a workpiece and the system includes a mechanism for generating asound wave in the workpiece, a polarizing interferometer for recordingsurface movement of the workpiece in response to movement of the soundwave and a mechanism for calibrating surface movement measured by thepolarizing interferometer relative to a workpiece property whereby theprocess is controlled by the measured property actually existing withinthe workpiece.

In accordance with a more specific feature of the invention theinterferometer (apparatus and method) includes a mechanism forgenerating a source of light, and an initial beam splitter for splittingthe light source beam into a reference beam and a signal beam which inturn is directed against the workpiece in the vicinity of the soundwave. A mirror arrangement is provided for collecting scattered lightfrom the workpiece as the signal beam and directing the signal beam ontoa signal beam path while the reference beam is also directed onto areference beam path. A mechanism is provided for polarizing thereference beam and the signal beam with at least one of the beamsgenerally circularly or elliptically polarized and an arrangement isprovided to combine the signal and reference beam into a resultantpolarized beam with one of the signal and reference beams which make upthe resultant beam having a predetermined phase-shift relative to theother. A photon detecting device senses light in the resultant beam andgenerates an electrical signal in response to the sensed lightindicative of the intensity of and the relative phase-shift of lightattributed to the signal beam and a calibrating arrangement is providedfor correlating the electrical signal with characteristics of the soundwave and in turn correlating the sound wave characteristics with aproperty of the workpiece so that the sensed wave movement defines aspecific property of the workpiece.

In accordance with another feature of the invention, a timingarrangement is employed to generate a plurality of sound waves each ofwhich is correlated in timed relationship to the electrical signalsgenerated by the photon detecting device to generate a plurality ofelectrical signals which in turn are conditioned to generate an averagesignal truly indicative of sound wave speed. In other words, the timingarrangement establishes the precise time at which the interferometerrecords a phase shift indicative of the arrival of the sound wave at apoint on the workpiece surface from which the time-of-flight of thesound wave is computed and compared to a reference signal or "look up"tables to ascertain the workpiece property such as temperature. Inaccordance with one important aspect of the invention, the signalconditioning arrangement comprises mechanisms for squaring each signal,summing the total of the squared signals and averaging the sum to arriveat an accurate average signal which dissipates adverse signal noiseeffects. Alternatively, the conditioning mechanism can simply comprise ascanning arrangement which discards unresponsive signals and generatesan average signal from a discrete subset of responsive signal.

In accordance with another significant feature of the invention,Rayleigh surface waves and longitudinal and/or shear waves areindividually sensed by the interferometer to provide two distinctelectrical signals. A mechanism is provided for calibrating each of thesignals to ascertain a sensed property of the workpiece. In accordancewith one aspect of the invention the sensed property determined by thecalibrating mechanism is the temperature of the workpiece so thatsensing both waves provides a measurement of the temperaturedistribution within the workpiece. In accordance with another aspect ofthe invention the sensed property is determined by comparing the sensedwave speed to a reference wave speed to determine desired physicaland/or metallurgical properties imparted to the workpiece during theindustrial process which a metal object would undergo, such as phasetransformations. In this connection the system could sense not onlytime-of-flight but also generate a signal indicative or correlatable tothe amplitude of surface movement.

In accordance with one particularly important aspect of the invention,the type of industrial process controlled by the invention herein, is ofthe heat treating type in which the workpiece is heated in an insulatedfurnace enclosure which inherently transmits to the workpiece a randomvibration having a speed and/or frequency less than that of theultrasound waves induced in the workpiece. The furnace enclosurecontains sight windows to permit mounting not only the interferometeroutside the heated enclosure but also mounting of the exciter lasergenerating the sound waves in the workpiece outside of the enclosure,thus obviating the adverse influences of heat on the mechanismgenerating the sound wave and sensing the movement of the workpiecesurface in response to the sound waves. Significantly, the randomvibrations permit discernible generation of electrical signalsindicative of phase shifts in the signal light beam so that only onephoton detector is required when a plurality of measurements arerecorded in a short time span to ascertain time-of-flight.

In accordance with an important feature of the invention, a focusingmechanism is provided in the signal beam path focusing the signal lightbeam on a limited spot on the workpiece to produce a large specklepattern indicative of the surface roughness of the object, and anadjustable aperture mechanism is situated in the path of the resultantbeam to focus on the photon collecting device a portion of the resultantbeam which essentially encompasses only one speckle in the granularspeckle pattern so that a diffusive surface of the workpiece does notinterfere with the measurement of the workpieces surface movement.

In accordance with yet another aspect of the invention, the mirrorarrangement establishing the optical reference beam path and the signalbeam path is adjustable to the extent that the paths are optically equalin length to a value within the coherence length of the light sourcebeam, thus permitting application to industrial processes in which theenvironment prevents precise adjustment of nonstable (i.e. vibrating)optical path lengths.

In accordance with another specific feature of the invention, anadjustable mechanism is provided to variably retard the plane ofpolarization of the linearly polarized light source beam prior tocombining the signal and reference beams into the resultant beam toachieve optimum light distribution between the signal and reference beamwhereby the surface motion of opaque workpieces can be measured. Stillfurther, the state of polarization of the signal beam is changed toelliptically polarized light and the retardation plane mechanism variedto produce in the resultant beam, a signal beam component which iscircularly polarized. A beam splitter mechanism may be provided in theresultant beam path to produce two orthogonal resultant beam projectionseach of which generates an electrical signal in photon collectingdevices and a calibrating arrangement is effective to generate onesignal indicative of the absolute difference in optical path lengthsbetween signal and reference beams.

In accordance with still another specific aspect of the invention, anoptical interferometer (apparatus and method) is provided which includesa source of light which is linearly polarized, a mechanism for splittingthe source of light into a signal light beam and a reference light beam,a mirror mechanism for forming a signal beam path along which the signalbeam travels to and from an object whose surface is to be measured and amirror mechanism for forming a reference beam path along which thereference beam travels with the reference beam path and the signal beampath being approximately optically equal in length and within thecoherence length of the light source beam. A mechanism is provided tochange the polarized state of one of the reference and light beams and amechanism is provided to combine the reference beam and the signal beaminto a resultant beam. A mechanism is also provided to split theresultant beam into two orthogonal, polarized light projections and aphoton detecting mechanism senses the intensity of each orthogonalproject and generates an electrical signal correlated thereto. Amechanism is then provided for conditioning the signals so that thedifference in optical path lengths between signal and reference beamscan be measured instantaneously and without introducing multiple phaseshifts into the reference light beam.

In accordance with still another specific aspect of the invention, anoptical interferometer is provided which includes a source of light, amechanism for splitting the source of light into a signal light beam anda reference light beam, a mechanism for forming a signal beam path alongwhich the signal beam travels to and from an object whose surface is tobe measured and a mechanism to form a reference beam path along whichthe reference beam travels, with the reference beam path and the signalbeam path approximately optically equal in length and within thecoherence length of the light source beam. A mechanism is provided forlinearly polarizing, in a vertical plane, one of the signal andreference beams and linearly polarizing the other one of the signal andreference beams in a horizontal plane. A combining mechanism is thenprovided to combine the reference beams and the signal beams into aresultant beam and split the resultant beam into two equal lightprojections. A first splitting arrangement is provided to split one ofthe two light projections into two, orthogonal, linearly polarized lightprojections and a second splitting arrangement is provided to circularlypolarize the other one of the light projections and split the circularlypolarized light projection into two orthogonal linearly polarized lightprojections so that each of the four light projections are shifted inphase 90° relative to one another. Photon detectors are then providedfor generating electrical signals for each orthogonal, linearlypolarized light projection which are correlated to the intensity andphase-shift of the signal beam and an arrangement for conditioning thesignals results in a measurement of the difference in optical pathlengths between the signal and reference beams from only oneinstantaneous reading thereby obviating the necessity of making severalphase-shifts in the reference beam and several measurements thereof todetermine the difference in path lengths.

It is a principle object of the invention to provide a system, methodand/or apparatus which can non-destructively measure sound waves inducedin a piece, such as a workpiece in an industrial heat treat or furnaceenvironment and to then, in accordance with the measurements obtained onthe work, gather information about the piece to control the industrialprocess.

In accordance with the principle object, the system, and/or apparatus ofthe present invention possesses any one or more of the followingspecific objects when compared to other known systems:

a) Remote detection and large standoff permitting remote installation,such as outside a furnace.

b) Simple and reliable optical system suitable for an industrialenvironment.

c) Beneficial rather than detrimental effect of external vibrations formeasurement of sound wave characteristics.

d) Excellent signal-to-noise ratio even for thick steel workpieces of3".

e) Ability to detect measurable time-of-flight and/or structuraldifferences of sound waves during heating of the work.

f) Ability to detect measurable time-of-flight and/or structuraldifferences in sound waves through workpieces in various physical ormetallurgical states.

g) Moderate cost of components.

It is a more specific object of the invention to provide a polarizinginterferometer which can be simply and cost efficiently constructed.

It is yet another object of the invention to provide a control systemfor an industrial heat process which uses laser induced sound waves inthe work and a polarizing interferometer to sense movement of the soundwaves in the work so that the work can be non-destructively measuredduring the process and the process regulated in accordance with themeasurements indicative of a property actually existing as of the timeof measurement in the work.

Yet another specific object of the invention is to provide an instrumentfor non-destructively measuring the bulk temperature, surfacetemperature and/or temperature distribution within a workpiece during aheat treat process.

Yet another specific object of the invention is to provide a polarizinginterferometer which can measure movement of the surface of a workpiecenotwithstanding vibration of said workpiece.

A still more specific object of the invention is the provision of apolarizing interferometer which utilizes random vibrations of aworkpiece to measure speed of sound waves in a workpiece so that onlyone photon sensing device is required to generate a responsive signalthus producing a cost effective device.

A still further specific object of the invention is to use a strobelight to generate the interferometer light source beam in the systemdescribed herein thus resulting in a cost effective interferometer.

Still another specific object of the invention is to provide apolarizing interferometer which can sense movement of diffusiveworkpiece surface notwithstanding the fact that such surface is sodiffusive that it exhibits speckle light patterns.

Still another object of the invention is to provide an interferometerwhich can measure generally opaque objects by means of an adjustablehalf wave plate that varies the light distribution between reference andsignal beams and to optimize the intensity of the electrical signal.

Still another specific object of the invention is to provide a controlsystem which uses either a sampling and/or a signal processing techniquein combination with an interferometer to accurately detect movement ofan ultrasound wave in a workpiece.

Still another specific object of the invention is to provide aninstrument which uses a laser to impart a sound wave to a workpiece anda polarizing interferometer to measure the shear, and/or surface and/orlongitudinal and/or shear ultrasonic waves resulting therefrom.

A still further object of the invention is to provide a system formeasuring the temperature and/or properties of metal work in industrialheat processes where such properties were not heretofore capable ofbeing directly measured in the work and controlling the process by suchmeasurements including, but not limited to, any one or more of thefollowing:

i) ion processes employing glow discharge techniques where the ion glowprevented workpiece temperature measurement;

ii) continuous strip processing such as galvanneal, galvanizing,aluminizing, etc., where strip emissivity prevented temperaturemeasurement;

iii) aluminum age hardening or annealing where metallurgicalcharacteristics were not previously measured;

iv) case depth heat treat hardening processes such as carburizing wherethe case depth could not previously be directly measured during theprocess;

v) heat treat processes which employ quenching and interrupted quenches;

vi) continuous casting and/or brazing processes in which the center ofthe casting or the brazing is molten when the work leaves the heat zone;

vii) sintering processes in which powder metal parts are densified andsimilarly ceramic process;

viii) surface treating processes involving various metallurgicalprocesses such as decarburizing, oxidizing, blueing etc.;

ix) processes involving composite materials or bi-metallic metals; and

x) in general processes which vary or control grain size or structure.

Still yet another object of the invention is to provide a phase shiftinginterferometer which is capable of measuring an object without having tointroduce multiple phase shifts to the signal or reference beam.

Still yet another object of the invention is to provide a mirrorarrangement in an industrial process permitting multiple locationmeasurements of sound waves traveling in a workpiece.

An important object of the invention is to utilize the sound wavetechniques of the present invention not only to control the workpieceprocess but also to monitor the integrity and soundness of furnacecomponents subjected to the heat environment, such as electric heatingelements, hearth components etc., to insure that the furnace isfunctioning properly.

In conjunction with the two immediately preceding features, it is stillyet another important object of the invention to utilize a mirrorarrangement, preferably a rotating and stationary mirror arrangement touse one workpiece analyzer to test furnace component soundness as wellas monitor workpiece performance or to monitor a plurality of zoneswithin a batch, continuous, or semi-continuous furnace or strip linei.e. time sharing type of arrangement.

Still yet falling within the broad object of the invention is the use ofan optical interferometer to measure not only the time of flight of asound wave in a workpiece to obtain information about the workpiece, butalso to measure the amplitude or amount of surface movement in theworkpiece attributed to the sound wave and to correlate not only thetime of flight of the sound wave but also the workpiece movementattributed to the sound wave to determine physical and/or metallurgicalcharacteristics of the workpiece.

Falling yet within a broad object of the invention is the utilization ofan interferometer to measure any wave motion in a solid object whichpreferably is in a thermal elastic state, but which in theory could bein its liquid state to determine a property of the object.

A still further object of the invention is to use the system disclosedherein to accept or reject manufactured parts or accept and rejectmanufactured parts which are subjected to stress testing such asvibrations by non-destructively measuring such parts. Still yet anotherobject is to provide a system which is capable of determining thecomposition, physical and/or metallurgical, of manufactured parts.

Still further advantages of the invention will become apparent to thoseskilled in the art upon reading and understanding of the followingdescription of the preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention make take form in certain parts in arrangement of parts, apreferred embodiment of which will be described in detail herein andillustrated in the accompanied drawings which form a part hereof andwherein:

FIG. 1 is a pictorial representation of a batch furnace fitted with thecontrol system of the present invention;

FIGS. 2A, 2B, and 2C are schematic illustrations of the positions of theimpulse and detecting lasers used in the invention;

FIG. 3 is a schematic illustration of some of the components of theinvention;

FIG. 4 is a schematic illustration of certain components of analternative embodiment of the invention not shown in FIG. 3;

FIG. 5 is a schematic illustration of the components of an alternativeembodiment of the interferometer of the present invention;

FIG. 6 is a schematic illustration of the components of an alternativeembodiment of the interferometer of the present invention;

FIG. 7 is a schematic illustration of the components of the preferredembodiment of the interferometer of the present invention;

FIG. 8 is a schematic illustration of the components of theinterferometer of the present invention modified to account for specklepattern resulting from diffusive surfaces of the workpiece;

FIG. 9 is a schematic illustration of the formation of speckle phenomenafrom a diffusive workpiece surface;

FIG. 10 is a graph showing the intensity of the photon multiplierelectrical signal output on the "Y" axis and the difference in lengthbetween signal and reference beam paths or surface displacement on the"X" axis;

FIG. 11 is a reproduction of a display of an "X-Y" mode oscilloscope ofa combination of two 90° apart electrical signals from the two photondetectors shown in FIGS. 6 and 8;

FIGS. 11A, 11B, 11C and 11D are "X-Y" oscilloscope displays similar toFIG. 11 but with the axis of the half-way plate rotated as indicated;

FIG. 12A is an electrical signal trace of a longitudinal and/or shear(surface) wave generated in a workpiece by an impulse laser and recordedby a piezoelectric transducer;

FIG. 12B is an electrical signal trace of a longitudinal and/or shear(surface) wave generated in a workpiece by a piezoelectric transducerand sensed by the interferometer of the present invention.

FIG. 13 is a plot showing surface displacement versus photon multipliersignal for two photon multipliers sensing light in a resultant beamwhich are 90° apart or 90° shifted in phase;

FIGS. 14A and 15A are electrical signal time traces of unprocessedphoton multiplier signals and FIGS. 14B and 15B are processed timesignal traces for FIGS. 14A, 15B respectively;

FIGS. 16A, 16B, 16C and 16D are schematic illustrations of the signalconditioning method employed in the invention. FIG. 16E is a schematicblock diagram of the signal conditioning scheme shown in FIGS. 16A, Band C;

FIGS. 17A, 17B and 17C are timed, electrical signal traces illustratingtime of flight of sound waves in a solid object at various temperatures;

FIG. 18 is an equilibrium diagram for an aluminum copper;

FIG. 19 are pictorial representations of grain patterns for variousmetallurgical reactions incurred in heat treating aluminum copper alloy;

FIG. 20 is a schematic illustration of a mirror arrangement for takingmultiple measurements of metal billets or slabs;

FIG. 21 is a schematic view of a galvanneal strip line withcorresponding strip temperature profile;

FIG. 22 is a graph of sound wave speed in homogenized andnon-homogenized aluminum at various temperatures; and

FIG. 23 is a graph of sound wave speed in carburized and non-carburizedsteel plate.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings wherein the showings are for the purposeof illustrating preferred and alternative embodiments of the inventiononly and not for the purposes of limiting the same, there is shown inFIG. 1, a control system for a batch-type industrial heat treat furnace10. The control arrangement includes (i) furnace temperaturethermocouple 12 for sensing the temperature of the furnace atmosphere,(ii) a gas arrangement, not shown but indicated schematically as arrownumbered 13 which controls the burner firing (i.e. the temperatureinputted into furnace 10) and optionally, the furnace gas composition,(as is well known in the art and which will not be described or shown infurther detail herein), (iii) the workpiece analyzer 15 of the presentinvention, which will be described further herein, and (iv) a centralprocessing unit 16. The computer or a central processing unit 16typically includes a high limit temperature instrument 17 and a floppydisk drive 18 for any particular program control. As generally indicatedschematically in FIG. 1, workpiece analyzer 15 inputs a process controlcharacteristic, typically temperature as indicated by arrow numbered 20and thermocouple 12 inputs the temperature of the furnace atmosphere asindicated by arrow numbered 21 into the central processing unit 16 whichwill use the information with appropriately developed, conventionalprograms to control the furnace gas atmosphere composition and furnacetemperature by signals outputted to gas control 13. Prior to theinvention, central processing unit 16 would extrapolate or empiricallydevelop workpiece temperature 20 to regulate gas control 13. Inaccordance with the invention the workpiece temperature or other sensedworkpiece characteristic is directly inputted to central processing unit16 for better control of the industrial process performed by batch-typeindustrial furnace 10. The programs do not per se form part of thisinvention in its broad scope. It is believed sufficient to simply notethat one skilled in the art would have no difficulty in modifyingexisting computer programs to directly input into the program theworkpiece temperature or other characteristic sensed by workpieceanalyzer 15 and control the process accordingly. Thus, disclosure of aprocess control program is not believed necessary to an understanding ofthe invention and is not disclosed herein.

FIGS. 2A, 2B and 2C illustrate how the two principle components of theinvention may be positioned relative to a workpiece 23 to measuremovement of a wave therein. The two components include a device which inthe preferred embodiment is an impulse laser 24 for generating a waveindicated by reference numeral arrow 25 which is sensed by a detectingdevice which in the case of the invention is an interferometer 26 andspecifically a polarizing interferometer.

One of the basic concepts of the invention is to use a polarizinginterferometer 26 to monitor the arrival of the sound waves at anappropriately selected spot on workpiece 23. The principle of thepolarizing interferometer operation is that any deflection of thesurface of workpiece 23 can lead to phase shifts of the interferometerbeams and the phase shifts can be detected as a change in thepolarization level of the interfering beams. Thus time at which impulselaser 24 is fired can be recorded and the time at which interferometer26 senses workpiece surface movement can be recorded to establishtime-of-flight of the ultrasound wave from which its speed can becalculated. In accordance with the preferred embodiment, the speed ofultrasound waves is a function of temperatures of the workpiece and thusthe temperature can be determined by using self-contained calibrationcurves or look-up tables. Time-of-flight as discussed hereafter can alsobe used to determine other characteristics, physical and/ormetallurgical, of the workpiece. However, the invention is not limitedin scope to only time-of-flight, but also can include measurement,relative or otherwise, of surface displacement of workpiece 23attributed to sound waves and correlating same to characteristics ofworkpiece 23, physical and/or metallurgical.

In accordance with the broad concepts of the invention, the waveproducing device need not be an impulse producing laser 24, but could bea conventional piezoelectric transducer or any other known device togenerate a wave in workpiece 23 although, in accordance with specificfeatures of the invention, an impulse producing laser such as a QuestechExcimer gas laser is preferred because it permits the instrument to belocated outside the furnace and can generate the desired waves withoutmarking or otherwise marring the surface of workpiece 26. Further, inaccordance with the preferred embodiment of the invention, impulse laser24 produces sound waves, specifically ultrasound waves, in workpiece 23.While in the preferred embodiment, ultrasound waves are utilized so asnot to be confused with external vibration and specifically ultrasoundwaves having frequencies higher than about 100 kH_(z), the invention isnot limited to ultrasound waves. The invention in its broaderapplications could in theory include waves other than sound waves andworkpiece 23 can in fact be in a liquid state. The detection device inaccordance with the invention is the interferometer 26 disclosed herein.FIG. 2A shows the relative position of impulse laser 24 andinterferometer 26 for measuring wave movement in thin workpieces such asstrip or thin plates whereat wave 25 generated in workpiece 23 is asurface wave such as lamb waves. FIG. 2B illustrates the position ofimpulse laser 24 and interferometer 26 for measuring reflected wavemovement in workpieces 23 which may be from 1/2" to several inchesthick. Wave 25 generated in workpiece 23 of FIG. 2B will includelongitudinal and/or shear as well as surface waves. FIG. 2C illustratesthe relative position of impulse laser 24 and interferometer 26 forthick workpieces 23 in which interferometer 26 measures bulk orlongitudinal and/or shear wave movement through workpiece 23. In thearrangement shown in FIG. 1, workpiece analyzer 15, for purposes ofillustration, is a FIG. 2B application. If the workpiece 23 being heatedwithin furnace 10 was a thick item, impulse laser 24 would be mounted onthe opposite side of furnace 10 from that of workpiece analyzer 15. Aswill be explained in further detail below, impulse laser 24 will operateto produce a series of wave pulses over a fixed time period. To avoidmarking or marring the surface of workpiece 23 during this time period,the point of impact of the laser beam will be slightly varied.Accordingly interferometer 26 may also be slightly moved to coincidewith movement of impulse laser 24 by means of a common mount such asshown in FIG. 3.

In verification tests, a Model 2220 Questech laser was used to generateultrasound waves from which the electrical signal traces disclosedherein were produced. Laser data is set forth below:

    ______________________________________                                        Gas:                   XeCl                                                   Wavelength:            308 nm                                                 Rated Pulse Energy:    200 mJ                                                 Actual Pulse Energy Used                                                                             80 mJ                                                  in Experiments:                                                               Max Repetition Frequency:                                                                            50 Hz                                                  Average Power:         8 W                                                    Pulse-to-Pulse Stability:                                                                            ±3%                                                 Pulse Duration (nominal FWHM):                                                                       10-25 ns                                               Beam Dimensions (V × H):                                                                       8-13 × 20 mm                                     Bean Divergence (V × H):                                                                       2 × 3 mr                                         Timing Jitter from External                                                   Trigger:               ±2 ns                                               ______________________________________                                    

Impulse laser 24 is used not only as an ultrasound generator but as atriggering source as well. As will be described with reference to FIG.16E, each time the laser fires a pulse onto the sample surface, a verysmall part of the beam is diverted into a fast response photomultiplierwhich is then used to trigger the data acquisition system (i.e.establish a starting point). A Tektronic 2430 oscilloscope (not shown)can be used as a data acquisition unit (10 nsecs resolution) which isinterfaced with central processing unit 16 where on-line processing ofthe data is preformed. It is contemplated that any energy source, lightfor example, such as that produced by a flash lamp (and morespecifically, a strobe light) can be used to replace the impulse laserfor cost consideration purposes.

Referring now to FIG. 3, a reflective wave (FIG. 2B) measurementapparatus is disclosed. In FIG. 3, workpiece analyzer 15 is entirelymounted within a workpiece analyzer housing 28 which contains a coolingfan 29 so that the electrical devices contained therein are notsubjected to excessive ambient temperatures. Workpiece analyzer 15 ismounted about 1 meter away from the work contained in industrial furnace10 which has conventional insulated furnace walls 30 which in turndefine an insulated furnace enclosure 31. Furnace enclosure 31 containsworkpiece 23 which is heated and in the process thereof heat treatprocesses are effected thereon. Within at least one furnace wall 30 isan infrared sight window 33 which permits viewing of workpiece 23 withinfurnace enclosure 31.

Within workpiece analyzer housing 28 is impulse laser 24 describedabove. An adjustable translation mount and lens 34 focuses impulse laserlight beam 35 to impinge a point 36 on workpiece 23, it being understoodthat impulse light beam 35 passes through sight window 33 and a window38 in workpiece analyzer housing 28. Focusing impulse laser light beam35 at point 36 produces sound waves 25 traveling through workpiece 23.Also mounted within workpiece analyzer housing 28 is a detecting laser40 which generates a monochromatic source light beam 41 which by meansof an adjustable translation mount and lens 42 passes through windows 44of an interferometer 45 and then through window 38 in workpiece analyzerhousing 28 and sight window 33 and furnace wall 30 before impingingworkpiece 23 at a point 47. Satisfactory results have been obtained withan He-Ne detecting laser. However, it is contemplated that any coherentlight to generate light source beam 41 which may be monochromatic can beused. As shown in FIG. 3, the distance "X" between the impingement pointof impulse laser beam 36 and source light beam 47 is established as afunction of the thickness "H" of workpiece 23, the intensity of theimpulse beam etc. in accordance with conventional and known theory. Asdiagrammatically shown in FIG. 3, source light beam 41 will afterimpinging point 47 on workpiece 23 scatter as indicated at 48, passthrough furnace sight window 33 and be collected by a collection lens 49and pass through a filter 50 before being directed into interferometer45.

Referring now to FIG. 4 there is shown in schematic form an arrangementof components within interferometer 45. All of the components areindividually well known and commercially available. As shown the lasersource beam 41 is split by a beam splitter 53 into a reference beam 54and a signal beam 55. A mirror 56 which is adjustable as at 57 isprovided in the path of reference beam 54 so that the optical pathlength of reference beam 54 is approximately equal to the optical pathlength of signal beam 55, and in fact reference beam path 54 and signalbeam path 55 are equal in length to within a value which does not exceedthe coherence length of light source beam 41. In the path of referencebeam 54 is a linear polarizer 59 and in the path of signal beam 55 isalso a linear polarizer 60. For consistency in terminology herein, theterm "state of polarization" or "polarizing state" refers to anypolarization of light, whether circular, elliptical, linear, or even ifthe plane of polarization is retarded. In the embodiment disclosed inFIGS. 4 and 5 one of the linear polarizer 59, 60 polarizes its lightbeam on the vertical axis while the other linear polarizer polarizes itslight beam on the horizontal axis. Linearly polarized reference beam 54and linearly polarized signal beam 55 are then combined in a beamcombiner 62 to produce a resultant beam 63 which is then passed througha polarizing beam splitter 64 which splits the resultant beam into twoorthogonal linearly polarized light beam projections 66, 67. Theintensity of orthogonal light beam projection 66 is recorded by photontube 68 and the intensity of second orthogonal light beam projection 67is recorded by photon tube 69. Electrical signals produced by photontubes 68, 69 are summed in a summing amplifier 70 to develop a differentsignal indicative of the phase-shift in signal beam 65 which is theninputted to central processing unit 16.

In the interferometer disclosed in FIG. 4, the interfering beams i.e.signal and reference beams 55, 54 are cross-polarized i.e. 59, 60 andthen combined at combiner 62 and their relative phase is determined bymeasuring the polarization of the resultant beam 63. Conceptually, thisis a demonstrated and convenient way to make such measurements. Thepoint is that if the surface of the workpiece moves, signal beam 55 willchange in phase relative to reference beam 54 and this will change thepolarization of their super-position in resultant beam 63. Referring tothe polarization conventions shown in FIG. 4, if the two beams i.e.reference beam 54, signal beam 55 are in phase, the super position ofboth beams will be polarized as indicated by the intensity signalgenerated at first photon multiplier 69. If the two beams are 180° outof phase, the resulting polarization will be the intensity signalgenerated at second photon tube 68. Looking at the difference betweenthe two signals i.e. electrical photon tube signal 68, 69 the effect ofmotion on the surface of workpiece 23 can be ascertained. The absolutephase difference is meaningless and is not measured in FIG. 4. Whatmatters is the rapid change in phase due to the sound wave in workpiece23. This approach eliminates the need to mount all the optics so thatthey stay fixed to within a wave length i.e. using heavy tables or steelbeams which are often associated with interferometric measurements. Allthat is required is to keep the reference and signal beam path lengthsapproximately equal and to keep the optics well enough in line toproperly combine the beams i.e. to being within the coherence length ofthe light source beam. So long as the external vibrations are at a lowerfrequency then one wishes to observe, there is no effect on phasechange.

It is to be noted that the intensity difference between the two measuredelectrical intensity signals in FIG. 4 i.e. the difference inintensities between photon tube 68, 69 varies as the cosine of therelative phase of the two beams. This means that the system has a maximaand minima and, at those phases, the system is insensitive to changes inphase. If, on a particular pulse, the intensity happens to be dividedabout equally between the two photon tubes 68, 69 because, the signaland reference beams 55, 54 were 45° or 135° etc. out of phase, one wouldobserve a strong signal from a small change. But if the phase happenedto be such as to put all the light to one of the two photon tubes 68,69, a small change in phase would have only a second order effect withno discernible signal. It is theoretically possible to address thisproblem in principle by controlling the relative path differencesperhaps via a feedback system on the reference beam mirror. A morefeasible solution to the problem is disclosed in FIG. 5 in which, notonly is the sensitivity issue addressed, but the absolute phasedifference between reference and signal beams can be recorded thusexpanding the use of the interferometer disclosed herein.

Referring now to FIG. 5 the reference beam 54 and signal beam 55 arecross-polarized i.e. horizontally and vertically and combined in apolarizing beam combiner 73. The polarized resultant beam 63 is thensplit into two with a 50% reflecting (non-polarizing) mirror 75 toproduce two equal beam components designated I_(A) and I_(B) in FIG. 5.The I_(A) component is split by a 45° polarizing beam splitter 76 intotwo orthogonal intermediate light projections 77, 78 and photonmultiplier 79 receives first intermediate beam projection 77 and secondphoton multiplier 80 receives second intermediate beam projection 78 todevelop electrical signals designated i₁ and i₂. These signals i₁, i₂may then be summed in a summing amplifier 80 to arrive at a differencesignal which is passed through a high pass filter 81 and squared as at82. The second resultant beam component designated as I_(B) in FIG. 5 iscircularly polarized by being passed through a birefringent quarter waveplate 84. The circularly polarized light beam is then passed through apolarizing beam splitter 85 which separates the beam into a right handcircular component 87 the intensity of which is recorded by right handphoton multiplier 88 and a left hand circular component 89 the intensityof which is recorded by left hand photon multiplier 90 to produce insystem "B" two photon multiplier signals relative to one another andalso shifted in phase 90° relative to system "A". The photon multipliersignals in system "B" and in system "A", are then summed in a summingamplifier 91 to produce a difference signal which is then passed throughhigh pass filter 92 and then squared as at 93. The signals for systems"A" and "B" are then added at summing amplifier 95 to give an electricalsignal i_(s) indicative of the absolute phase-shift. In prior art,polarizing interferometers a quarter wave plate in the reference beam isrotated by means of an encoder through various phase angles to generatepredetermined phase-shifts which in turn generate multiple readingswhich are then algebraically resolved to determine an absolutephase-shift from which the distance between the signal and referencebeams can be calculated. As indicated above, the time constraintsrequired to shift a quarter wave plate would preclude use of phaseshifting interferometers for sensing sound wave displacement. Inaccordance with the system shown in FIG. 5 absolute measurement surfacedisplacement is possible from one instantaneous reading. While it is theprimary objective of this invention to measure time of flight of soundwaves within workpiece 23, nevertheless it is contemplated as fallingwithin the scope of the present invention that in ascertaining certainphysical or metallurgical properties of workpiece 23, the amplitude orsurface displacement of workpiece 23 attributed to sound wave 25 may, incertain instances, be measured and correlated to a known wave amplitudeto determine when the desired workpiece property or characteristic hasbeen achieved. The system illustrated in FIG. 5, as are the othersystems subsequently discussed herein, is capable of recording suchmeasurements. In other words, the systems are capable of operating aspolarizing interferometer per se but with the added advantage ofobtaining a measurement from one reading.

The mathematics demonstrating the soundness of the system disclosed inFIG. 5 is set forth at pages 34-42 of the NTIS report incorporatedherein by reference. Comparing the system disclosed in FIG. 5 to thatdisclosed in FIG. 4 it is again noted that the signal and referencebeams are cross-polarized and then combined as before, but the combinedbeam is then split into two with a 50% reflecting (non-polarizing)mirror. One of these is then separated into the two intermediate linearpolarization components as done in FIG. 4, while the other is passedthrough a birefringent quarter wave plate and then split into twointermediate linear components--a sequence that separates the twocircular components of the original beam. Both pairs of intensities aredifferent, the two difference signals are passed through high passfilters and then squared and finally the two are added. To obtain aresult comparable to the output of the FIG. 4 system, the square root ofthe final signal can be taken. A bit of algebra may clarify the effectof all of this. If the relative phase of the signal and reference beamsis denoted "p", then the two intensity difference signals areproportional to cosine (p) and sine (p). Going through a high passfilter is like taking the derivative, so the signals after the filtersare -(dp/dt) sin (p) and (dp/dt) cos (p). When these are squared andadded, the result is just proportional to the square of dp/dt which isthe change in phase between the signal and reference beams--that is themotion of the surface of the piece due to the sound wave.

The interferometer system disclosed in FIG. 5 is simplified and improvedupon in a cost reduction sense by the alternative embodiment disclosedin FIG. 6 which in turn is still further simplified and improved upon ina cost reduction sense by the preferred embodiment interferometer systemdisclosed in FIG. 7. It is also to be noted that the present inventionevolved from the interferometer system disclosed in FIG. 5 to theimproved upon system disclosed in FIG. 6 and finally to the preferredembodiment disclosed in FIG. 7, and while all systems disclosed in FIGS.5, 6 and 7 measure phase-shifts in the signal beam the manner in whichthe phase-shift is measured is different, and conceptually differentbetween the systems illustrated in FIGS. 5, 6 and 7.

In the interferometer system disclosed in FIG. 6, light source beam 41from detecting laser 40 (or alternatively a strobe light) is linearlypolarized by polarizer 96 and the linearly polarized light is split by areflective beam splitter 97 into a signal beam 55 and a reference beam54. Because of the opacity of workpiece surface 23 the intensity oflight source 41 is split such that 95% of light source 41 travels assignal beam 55 while 5% of light source 41 comprises reference beam 54.This split could vary to 90%--signal beam and 10%--reference beam.Signal beam 55 is focused by means of lens 98 onto the surface ofworkpiece 23 and then to a non-polarizing beam combiner 99. Thereference beam 54 travels along an adjustable mirror 56, 57 path to ahalf wave plate 100 where its plane of polarization as shown by thepolarization symbol in FIG. 6 is retarded and then reference beam 54passes through a quarter wave plate 101 whereby reference beam 54 iselliptically polarized. Elliptically polarized reference beam 54 andlinearly polarized signal beam 55 are combined in non-polarizing beamcombiner 99 to produce a circularly polarized resultant beam 63.Resultant beam 63 thus comprises a circularly polarized reference beam54 and a linearly polarized signal beam 55 and the resultant beam isthen split by a 45° polarizing beam splitter 103 into two orthogonalpolarizations, namely a first beam 104 the intensity of which ismeasured by first photon multiplier 105 (could also be a photodiode),and a second beam 106 the intensity of which is measured by secondphoton multiplier 107 (could also be a photodiode). This action leads toa 90° phase difference between the two orthogonal polarizations of thereference beam which was circularly polarized before the beam combiner99. No phase-shift between orthogonal polarizations takes place forlinearly polarized signal beam 55 only. Any phase change between signalbeam 55 and reference beam 54 will change the distribution of lightintensity between first and second photon multipliers 105, 107 and henceprovide a signal. The phase changes will occur if the target orworkpiece surface moves due to the arrival of ultrasonic waves at thebeam impingement point.

FIG. 10 is a graph of a photon tube signal output plotted on the "Y"axis and the surface displacement plotted on the "X" axis, i.e. thedifference in length between reference beam path and signal beam path.In verifying the interferometer design of the invention, a round glasswheel with pie shaped sectors having various wedge angles (thickness)was inserted into the path of signal beam 55 between lens 98 andworkpiece 23 so that the optical path of signal beam 55 would be altereda known distance equal to the wedge angle of each pie shaped sector asthe glass wheel rotated in the path of the signal beam withintersections between adjacent pie shaped sectors clearly indicated by"blips" in the signal output trace. FIG. 10 is an idealized view of aportion of the signal output trace generated for a portion of one piesector of the glass wheel (peaks not being shown). Each complete cyclein the signal output (which as noted above is a cosine wave) correspondsto a displacement of one wave length (for He-Ne light, this is 632.8 nm)and by counting the number of cycles very precise quantitativeinformation for the thickness change in each pie sector can be obtained.Obviously the steeper the wedge the more number of cycles and theverification test established excellent agreement with staticconventional interferometer tests verifying the thickness of each pieshaped sector. Also, as the speed of the glass wheel increased theamplitude of the cosine curve decreased and is an indication of thefrequency response. Thus, absolute quantitative information can bedetermined from the output of either photon multiplier 105, 107.Importantly, the photon multiplier signal output curve of FIG. 10 showsthat the cosine signal output generated from photon multiplier 105 lags90° behind the cosine signal output generated by second photonmultiplier 105. Now referring to the discussion above, there is a maximaand a minima at which the sensitivity of any one of the photonmultiplier signals is maximized or minimized. If the system is operatedfor example at a point or time where second photon multiplier 107 signalis at point "A" a maximum sensitivity signal is obtained. If on theother hand the system is operated so that second photon multiplier 107output signal is at a peak value of the cosine curve, such as at point"B", the signal is no longer sensitive to small displacement changes.Because first photon multiplier 105 is shifted in phase 90° relative tosecond photon multiplier 107, then, as shown by the curve in FIG. 10should second photon multiplier 107 be at point "B", a condition oflimited sensitivity in small displacement changes, then first photonmultiplier 105 will be at point "C" a condition which is maximallysensitive to displacement changes. Thus, second photon multiplier solvesthe problem of sensitivity and also permits the system to operate as aninterferometer per se since the system is always sensitive to phasechange no matter what the particular mode of operation. In addition, theuse of a second photon multiplier will account for any variation insudden intensity changes in the light source beam although it should benoted that intensity changes in the light source beam will constitute"in phase" changes which the system will to some extent automaticallycorrect therefore since any change in light source beam intensity willappear also in the reference beam and the system is measuring only therelative phase shift between the reference and the signal beam. Thus,while there are benefits to the use of two photon multipliers, it is oneof the underpinnings of the present invention that only one photonmultiplier need be employed.

Referring now to FIGS. 11, 11A, 11B, 11C, and 11D, there is illustrateda display of an X-Y mode oscilloscope in which the "X" axis representsthe signal from one of the photon multipliers 105 and the "Y" axisrepresents the signal of the other photon multiplier 107. When thesystem is properly aligned so that the intensity of the electricalsignals i.e. i₁ and i₂ are equal then the two signals will form a circleas shown in FIG. 11. This indicates that the signals are exactly 90° outof phase. A signal is formed because of random vibrations at lowfrequency. No such circular signal is generated when the system isresting in an absolute calm environment and, in that case, a singlepoint (not a circle) in the X-Y plane represents the signal. In thealternative embodiment discussed above in connection with FIG. 6 and,for purposes of obtaining absolute measurements, circularly polarizedlight is to be generated in resultant beam 63 since otherwise,polarizing beam splitter 103 will not produce projections shifted inphase 90° to one another. It was determined that because of less thanperfect optical performance occurring in interferometer opticalcomponents such as the beam combiner, the polarizer etc. that circularlypolarized light produced by quarter wave plate 101 would undergoelliptical distortion and the output signals resulting from photonmultipliers 105, 107 would not necessarily be shifted 90° in phase.Investigation revealed that insertion of half wave plate 100 wouldretard the plane of polarization resulting in elliptically polarizedlight leaving quarter wave plate 101 so that the optical distortionproduced by the other elements in the beam path would, in effect,correct the ellipticity of the resultant beam so that it is circularlypolarized when it is split into its orthogonal projections. Morespecifically, it was found that by rotating the axis half wave plate 100the degree of ellipticity produced in reference beam 54 could beprecisely controlled. This is shown by the oscilloscope tracesillustrated in FIGS. 11A, 11B, 11C, and 11D in which the axis of halfwave plate 100 is rotated so that the plane of polarization was retardedby -10° in FIG. 11A, -20° in FIG. 11C, +10° in FIG. 11B and +20° in FIG.11D. Thus, in the alternative embodiment disclosed in FIG. 6 the systemis set up by shifting the axis of half wave plate 100 to produceelliptically polarized light in reference beam 54 to result incircularly polarized light in resultant beam 63 so that photonmultipliers 105, 107 record electrical signals equal to one another andshifted in phase 90°.

Referring now to FIGS. 8 and 9, all surfaces of any workpiece 23 have aroughness as indicated in FIG. 9 which is depicted in scale reference indistance to that of a wave length of light. When nearly monochromaticlight is reflected from such a surface, the optical wave resulting atany moderately distant observation point 110 consists of many coherentcomponents or wavelets, each arising from a different microscopicelement of the surface. With reference to FIG. 9, the distancestravelled by these various wavelets may differ by several or many wavelengths if the surface is truly rough. Interference of the dephased, butcoherent wavelets results in a granular pattern of intensity that iscommonly referred to as "speckle". This peculiar granular appearance orspeckle pattern of an object viewed in highly coherent light results ina granularity or speckle patent pattern which bears no obviousrelationship to the macroscopic properties of the illuminated object butrather appears chaotic and unordered with an irregular pattern that isbest described by the methods of probability theory and statistics. Themodification to the interferometer of the present invention toaccommodate the speckle pattern is based on the recognition that eachspeckle or light colored grain in the speckle pattern in resultant beam63 contains an interference pattern indicative of the phase-shiftbetween signal and reference beams 55, 54. The addition to the system ofFIG. 6 which accommodates the speckle pattern is shown in FIG. 8 toinclude a camera lens 111 (Cannon 100 mm focal length F2) which helpsfocus the beam onto a diffraction limited spot on the diffusive surfaceof workpiece 23. This focuses in on the speckle or light colored grainsappearing in the granular speckle pattern. First expanding lens 113 isprovided in the path of first polarized beam 104 and a second expandinglens 114 is provided in the path of second polarized beam 106 andexpanding lens (-100 mm focal length) 113, 114 are focused so that onlyone speckle or grain is passed onto first and second photon multipliers105, 107. This concept has proven effective to permit phase-shiftmeasurements attributed to sound waves in workpieces having rough,diffusive surfaces.

Referring now to FIG. 7 a preferred embodiment of the interferometer ofthe present invention is disclosed which has the advantage of being lesscostly to manufacture than the systems disclosed in FIGS. 5 and 6because, among other things, only one photon multiplier and correctivelens aperture is required. However, and perhaps more importantly, thesystem disclosed in FIG. 7 can be adjusted to maximize utilization ofsignal beam light such that detecting laser 40 could be replaced by astrobe light. In addition, the arrangement disclosed in FIG. 7 minimizesoptic distortion.

In FIG. 7, light source beam 41 is linearly polarized in polarizing beamsplitter 116 and the plane of polarization retarded by adjustablehalf-way plate 100 as discussed above. Source light 41 after passingthrough beam expander 109 including a focusing lens passes throughpolarizing beam splitter 117 where it is split into a linearly polarizedreference beam 54 having a retarded plane of polarization and a signalbeam 55 focused by lens 98 onto workpiece 23. Signal beam 55 passesthrough a quarter wave plate and on its return is elliptically polarizedover that portion of its beam path prior to becoming linearly polarizedwhen it passes again through polarizing beam splitter 117. Otherequivalent arrangements can use an eighth wave plate. Linearly polarizedsignal beam 55 and linearly polarized reference beam 54 are thencombined in a non-polarizing beam splitter or combiner 118 intoresultant beam 63 which passes through adjustable aperture 110 to focuson photon multiplier 105 to develop the electrical signal i₁ discussedabove. By varying the plane of polarization through half way plate 100,the ellipticity of signal beam 55 is likewise altered so that when itreturns through polarizing beam splitter 117 its linear plane ofpolarization can be established to increase the intensity or tosensitize the intensity of resultant beam signal 63. This of courseresults in a decrease in intensity of orthogonal signal leavingnon-polarizing beam splitter 118 which strikes set-up screen 120.Further, if a second photon multiplier 107 were substituted for screen120 so that absolute measurement changes could be obtained, then halfwave plate 100 would be adjusted in the manner described for FIG. 6 toproduce two equal intensity beams from beam splitter 118. In FIG. 7, abeam expander 109 (Newport Model LC1) in combination with focusing lens98 increases the size of the granular pattern or speckle and adjustableaperture 110 (NRC IRIS) is adjusted so that only one speckle or grain ispassed through to first photon multiplier 105.

Referring now to FIG. 13 there is shown a plot of photon multipliersignal on the "Y" axis versus surface displacement on the "X" axis fortwo photon multipliers sensing light in a resultant beam which are 90°apart or 90° shifted in phase. These curves represent response curves tolow frequency external or environmental vibrations imparted to workpiece23. As with FIG. 10, the points A, B, C and D on the curve for photonmultiplier no. 1 represent different operating points of theinterferometer system disclosed in FIG. 6 or 7. If the interferometerhappens to be at points A or B then its sensitivity is close to zerosince any deviation (due to surface displacement) from these points willnot produce any change in the photon multiplier signal i.e. "Y" axischange. On the other hand, at points like C and D, the sensitivity ismaximum and small surface displacements lead to large photon multipliersignals. In practice, the ultrasound waves are a small high frequencyripple which are superposed on the large, slow vibrations reflected inthe curve shown. It is points like C and D that make the signalfluctuate as described hereafter. This action is shown by the arrows atC and D in FIG. 13. If the system operates at C then a smalldisplacement would generate a positive signal, while if the systemoperates at point D, a similar displacement will lead to a negativesignal. The choice of operating point is purely random and cannot bepredetermined in an environment with unavoidable external vibrations. Atthe same time however because of the external vibrations, the operatingpoint of the interferometer will vary over a number of signals which canthen be conditioned through a signal conditioning scheme which will beshortly described. On the other hand, the use of a second photonmultiplier can avoid the problem of low sensitivity points A and B sincethose points correspond to the high sensitivity points E and F of thesecond photon multiplier. Thus, a smart software package can beimplemented which would switch from one photon multiplier to the otherand always pick that photon multiplier with high sensitivity reading.This is a specific advantage of the interferometer system disclosedherein over conventional interferometers where the detector must alwaysbe at points C and D for high sensitivity.

In the current system, only one photon multiplier is needed because asignal processing technique has eliminated the problems discussed above.The general processing scheme is conceptually illustrated in FIGS. 16A,B and D and schematically illustrated in block form in FIG. 16E.Referring first to FIG. 16E, a conventional triggering circuit 121 (suchas a Schmidt trigger) controls the firing of impulse laser 24, which asnoted varies its impulse position on workpiece 23 so that the same spotis not consistently impinged, and this in turn may result in acorrelating adjustment by detection laser 41. As noted above a portionof the beam of impulse laser 24 is directed to a fast response photonmultiplier 26, such as a type RCA 4526, which in turn establishes areference start time and signals photon multiplier 105 RCA 4526 (couldalso be a photodiode such as United Detector Technology Model No. PIN-10DP) to record the electrical signal. The electrical signal is digitizedin an analog to digital device 122, squared at 123, summed at 124 andthen averaged at 125. Its output 127 is then fed into an oscilloscopeand from there into central processing unit 16. The electrical circuitryfor performing these functions does not per se form part of theinvention, and the circuitry to perform such functions as described isbelieved well within the scope of a skilled electrical technician orengineer and thus is not shown or described in detail. Graphically thesignal conditioning scheme is shown in FIGS. 16A-D. FIG. 16A correspondsto the pulse signal recorded at point C in the graph of FIG. 13 and FIG.16B shows the negative pulse signal recorded at point D in the graphshown in FIG. 13. If the signals are simply averaged no discerniblesignal will result as shown in FIG. 16C. However, if the signals aresquared, summed and then averaged a truly average signal of the pulse ofthe sound wave over a fixed time span can be obtained as shown in FIG.16D. However, it is contemplated that many installations will operate atonly 32 or 16 pulses. The time over which the pulses are taken is afunction of the frequency of impulse laser 24 and the speed ofultrasound waves. For example if laser 24 generates at 50 Hz, that is a20 msec time interval between pulses, then the time-of-flight must beless than 20 msec to avoid overlapping, confusion etc. A typicalmeasurement of 128 pulses takes about 15 seconds and if the pulse numberis reduced to 16 or 32, the processing time would be reduced to a coupleof seconds. By processing and combining all of these pulses, the systemavoids low sensitivity regions and achieves an excellent signal noiseratio. Again, it is a random external vibrations imparted to theworkpiece which varies the points at which the interferometer operatesso that a plurality of pulses can be square average to produce a trulyaccurate signal which permits time of flight as well as wave amplitudeto be measured and correlated to other properties. This signalconditioning scheme is based on processing and combining a number ofpulses typically as many as 128.

FIGS. 14A, 14B and 15A and 15B illustrate the signal conditioningarrangement discussed with reference to FIGS. 16A-E. The signals inFIGS. 14A, 14B and 15A and 15B were obtained on 1" thick mild steelsamples at room temperature. In FIG. 15A the arrival of the ultrasonicwave cannot be detected in the unprocessed signal. This signal, i.e.FIG. 15A, represents a snap-shot of the data during a single ultrasonicwave pulse. However, when a number of pulses are acquired which were 128for the sample illustrated, the signal appears in FIG. 15B with adramatically improved signal noise ratio. FIG. 14A illustrates asnap-shot of a single wave pulse where the arrival of the wave can bedetected without signal processing. In fact, it is possible, for reasonsdiscussed above, to simply scan the sample of snap-shots and discardunacceptable readings such as FIG. 15A and average detectableunprocessed signals such as that discussed in FIG. 14A to arrive at areasonably accurate time of flight average. However, when the signalconditioning scheme is applied even to detectable unprocessed signalssuch as shown in FIG. 14A, the process signal shown in FIG. 14Bsignificantly improves the clarity of the signal. This improvement orclarity becomes especially important when more than one wave motion isbeing sensed.

The invention disclosed herein was initially developed to directlydetermine temperature measurement of workpiece 23 by determining time offlight of ultrasonic waves in the workpiece. FIGS. 17A, 17B and 17Cillustrate the temperature effect on time of flight of ultrasound wavesthrough 1" thick SAE Grade 8620 carbon steel with the time point ofultrasound generation to the left of the traces (not shown). FIGS. 17A-Cshow that the time of flight of the ultrasonic waves increases as thetemperature rises and makes it possible to correlate the temperature ofthe workpiece to look up tables or to a base reference signal. Tracessimilar to those shown in FIGS. 17A-C apply to other metals. Thispermits the heat treat process conducted in the batch furnace 10 to becontrolled by the temperature directly measured in the workpiece.

As noted at the outset, the temperature measurement is not limited tomeasurements of the bulk temperature of the workpiece or to measurementsat the surface of the workpiece. FIG. 12A is an uncorrected signalrecorded by the interferometer system disclosed in FIG. 6 or 7 of anultrasonic wave generated by a piezoelectric transducer. The uncorrectedphoton multiplier's electrical signal clearly show the wave arrival withlongitudinal and/or shear wave indicated by reference numeral 130 andthe wave arrival point of shear waves indicated by reference numeral131. FIG. 12B is the electrical signal trace developed by apiezoelectric transducer of an ultrasonic wave generated in a workpieceby impulse laser 24. Thus FIG. 12B demonstrates that an impulse lasercan generate ultrasonic longitudinal and/or shear waves 130 and shearwaves 131 which can be detected. Thus it is possible to simultaneouslymeasure both the bulk temperature of the workpiece through longitudinaland/or shear wave time of flight analysis and also the surfacetemperature of the workpiece through analysis of the Rayleigh or surfacewave time of flight movement such as by a laser/interferometerarrangement illustrated in FIG. Many heat treat processes requirethrough heating of the workpiece to achieve desired metallurgical andphysical properties. As noted above, the present practice is to simplyallow the workpiece to equilibrate to the desired temperature over apredetermined time which requires maintaining the heat throughput fortime periods longer than that which is metallurgically required. Thuswith the invention it is possible to precisely determine whenequilibration occurs to reduce the overall process time or alternativelyto ramp the furnace temperature to a higher temperature to achieve thebulk temperature in a shorter time period.

As discussed above, it is intended that the invention not only measuretemperature but also physical and metallurgical characteristics of theworkpiece. That this can be accomplished is illustrated first in FIG. 22which plots time of flight measurements recorded by the invention fornon-homogenized 1" aluminum 6061 bars as shown by line 135 andhomogenized 1" aluminum 6061 bars as shown by line 136. The differencebetween graphs 135, 136 is attributed to the degree that the aluminumhas been homogenized. To a similar effect is FIG. 23 which shows time offlight delay as a function of temperature for 8620 carbon steel in an asreceived condition indicated by graph 138 and for 8620 carburized steelindicated by graph 139. FIGS. 22 and 23 demonstrate that the heat treatprocess depicted by the curves i.e. homogenizing and carburizing can becontrolled by a "signature profile" control scheme. Reference should behad to Surface Combustion U.S. Pat. No. 4,193,069 for discussion ofprocess control by heat signature profiling, and U.S. Pat. No. 4,193,069is incorporated by reference herein in this respect. A similar techniquewould be employed to effect process control in the present invention inthat a time of flight profile or operating curve would be programmedinto central processing unit 16 which would be correlated to the graphsin FIGS. 22 and 23. Gas control 13 would then be regulated i.e. heatinput and furnace gas composition, so that the time of flightmeasurements maintain the profile of the signature time of flight graphprogrammed into the central processing unit. Prior to start of theprocess, initial time of flight measurements on the workpiece would betaken for calibration purposes relative to the signature profile orimprinted into the central processing unit. Again, since the time offlight measurement is an exact measurement, what the workpieceexperiences during the process is controlled directly by the propertiesand temperature actually existing within workpiece 23.

It is also within the scope of the present invention to control heattreat processes in which the metal undergoes phase transformation. As ofthe date of this application investigation of wave characteristicssensed by the interferometer system disclosed herein is continuing butinitial investigations indicate that the system has ability to detectphase transformations and control heat treat process accordingly. Inthis connection, initial investigations have been limited to time offlight measurement, but it is contemplated that actual measurement ofthe sound pulse movement i.e. two photon tubes in the system disclosedin FIG. 7 may conceptually be employed to provide additional indiciai.e. wave amplitude as well as wave speed for process control. However,it is believed that time of flight measurements and signal conditioningschemes will be sufficient to distinguish phase transformation.

As an example of one practical application of this concept reference maybe had to FIGS. 18 and 19. FIG. 18 is a graph of an aluminum-copperequilibrium diagram. Solution treatment of aluminum-copper alloycontaining about 5% copper is depicted in FIG. 18 and it should be notedthat along line 140, the temperature band for a single phase (k) solidsolution is narrow and the temperature in this narrow band (+5° F.) mustbe closely controlled to achieve uniform bulk temperature of theworkpiece if product quality must be achieved. Since the invention canmeasure surface and bulk temperature, excessive soak time can beeliminated from the process cycle time resulting in increasedproductivity. FIG. 19 are pictures or representations of grain structurefor age hardening of aluminum i.e. see FIG. 18. In the metallurgicalprocess termed age hardening or precipitation hardening, an aluminumalloy is initially heated to form a single phase solid solution i.e.grain pattern "A" shown in FIG. 19. It is then quenched so that thesolid solution is retained i.e. grain pattern "B" and reheated i.e.annealed to form grain boundary precipitate i.e. grain pattern "C". Themetal is then held at this intermediate temperature whereat agehardening or sub-microscopic precipitation starts i.e. grain pattern"D". The time of age hardening temperature is a critical factor.Insufficient time does not achieve desirable hardness properties and toomuch time at age hardening temperature causes the alloy to soften due togrowth of participate particles i.e. grain pattern "E". It is believedthat the ultrasonic wave patterns produced in the granular structuresindicated as "B", "C", "D" and "E" in FIG. 19 will vary and will permitdirect control of the process as a function of the precipitatehardening.

FIG. 1 illustrates a batch type industrial process. FIG. 21 illustratesworkpiece analyzer 15 applied to a strip line and more particularly to agalvanized strip line 150. This strip line performs a galvannealingprocess which is a transformation of a zinc coating into an alloyedcoating consisting of various iron-zinc alloy phases. In order to obtaina ductile coating without powdering, brittle phases in the iron-zincphase diagrams should be limited. Various parameters such as aluminumcontent of the coating, processing temperature and time, coatingthickness, etc. need to be considered for controlling the system.Galvannealing is a continuous process with a typical processing rate ofabout 50 tons/hour and process temperature ranges from 850°-1100° F.

In a galvanneal system incoming strip passes through a molten zinc pot151 then through a furnace 152 for heating and finally a cooler 153.Strip 150 gets coated with a thin layer of zinc in zinc pot 151 and whenit passes through furnace 152 the zinc transforms into various alloyswith iron. Furnace 152 has a heating zone 155 and a holding zone 156 toprovide adequate time for diffusion of iron into zinc for transformationreactions. Typical residence time for the strip in holding zone 156 isabout 15 seconds. After the strip is soaked a desired time, the strip iscooled in a cooling zone 153 to a temperature below 750° F. so that thestrip can come into contract will mill rollers (not shown) which guidethe strip. The heating process is intended to cause a diffusion of theiron in the base metal through the zinc coating and results in a mattesurface finish which is more desired for painting than standardgalvanized coating surface.

Alloy coatings present a dull surface texture which readily takes paintbut the alloy must be uniform so that no brighten unalloyed regionsremain. At the same time, over alloying must be avoided. Thus time andtemperature control play a significant role in the galvanneal process.Presently, sophisticated computer algorithms are used to set theseparameters. The emissivity of bright zinc coating is low but emissivityof low alloyed coating is high and therefore radiation pyrometers arenot suitable for temperature measurement and contact thermocouplescannot be used for this application because of surface damage. Thetransition band from bright zinc coating to a dull alloyed coating isnarrower in length and fluctuates along the length of the strip. Atpresent, this problem is addressed by visual observation of the band byan operator and adjusting the operating parameters manually whichrequires constant operator attention. The workpiece analyzer 15positioned as shown is ideal for control of this application because itis not affected by strip emissivity. More particularly, strip control byworkpiece analyzer 15 will permit an increase in the heating rate in theheating zone 155 with the result that the height of heating zone 155 canbe reduced with an overall reduction of the overall height of the line.Decreasing the line height reduces the length of the unsupported stripwhich reduces clearance requirements around the strip and the amount ofcold air entrained within the strip thus producing better product whilepermitting faster line speed. Similar considerations apply to otherstrip line processes such as annealing of sheet steel.

In addition to the batch heat treating processes performed in the batchfurnace of FIG. 1 and the strip line heat treating applications such asdisclosed in FIG. 21, there are also continuous or semi-continuousprocesses in which slabs, billets and the like are heated or reheated inelongated furnaces, a cross-section of which is illustrated in FIG. 20.In such furnaces, a metal slab 160 rests on furnace rolls 161 while itis heated from above and below and conveyed through various furnacezones in which the furnace atmosphere and temperature is closelycontrolled. Uniformity of the heat inputted to the slab from side toside is critical and FIG. 20 illustrates an application where oneworkpiece analyzer 15 using a FIG. 2B mount can measure the temperatureof the slab 160 from side to side. In the arrangement depicted severalsight ports 163 are provided in the top of the furnace and a rotatingmirror 164 directs the impulse laser and detector laser beams eitherthrough the center sight window or by means of fixed mirror 165 throughone or the other end sight ports 163. Thus one workpiece analyzer 15 canbe employed to scan transverse temperature measurements across slab 160.

The invention has been described thus far with specific reference tocontrol of industrial heat processes in which temperature of theworkpiece plays a particularly critical role in process control. Theinvention however is not limited to heat treat process control. Time offlight of sound waves varies through solids depending upon themetallurgical composition of the solid. Thus the workpiece analyzer canbe used in countless installations to nondestructively test amanufactured part at ambient temperature to control the manufacturingprocess i.e. castings. Time of flight of sound waves also variesdepending upon the soundness of integrity of the manufactured part.Workpiece analyzer can function to non-destructively test the soundnessof any solid manufactured item for acceptance and rejection of the part.More specifically however workpiece analyzer is ideally suited to testmanufactured items which are subjected to stresses from vibrations. Thatis, many industrial processes require the manufactured part to besubjected to external vibrations for life tests. Clearly, the workpieceanalyzer has application with the significant benefit that the part doesnot have to be destroyed. Thus in the aircraft industry it is nowpossible for each part, such as turbine blades, helicopter rotors,loading gear assemblies, etc. to be thoroughly tested in anon-destructive manner.

The invention has been described with reference to a preferredembodiment and alternative embodiments. Further alterations andmodifications to the invention will become obvious to those skilled inthe art upon reading and understanding the specification hereof. It isintended to include all such modifications and alterations in so far asthey come within the scope of the invention.

Having thus defined the invention, it is claimed:
 1. An opticalinterferometer comprising;a) a source of light generating a light sourcebeam which is linearly polarized; b) means for splitting said lightsource beam into a signal light beam and a reference light beam; c)means for forming a signal beam path along which said signal beamtravels to and from an object whose surface is to be measured and meansto form a reference beam path along which said reference beam travels,said reference beam path and said signal beam path being approximatelyoptically equal in length and within a coherence length of said lightsource beam; d) means to change the polarized state of one of saidreference and said signal beams; e) means to combine said reference beamand said signal beams into a resultant beam; f) means to split saidresultant beam into two orthogonal, polarized light projections; g)photon detecting means for sensing the intensity of each orthogonalprojection and generating an electrical signal correlated thereto; h)means for conditioning said signals to measure the path differencesbetween said signal beam and said reference beam from one lightinterference pattern; and dual use optical means for compensating foroptical imperfections in said interferometer to establish apredetermined phase relationship between said signal beam and saidreference beam and also to establish a predetermined intensityrelationship between said signal beam and said reference beam.
 2. Theinterferometer of claim 1 wherein said dual use optical means includes ahalf wave plate and means for varying an axis of said half wave plate tocause said means to change the polarized state to produce circularlypolarized light.
 3. The interferometer of claim 1 wherein said means tochange said polarized state is within said reference beam path.
 4. Theinterferometer of claim 1 wherein said means to change said polarizedstate is within said signal beam path and further including means forchanging the plane of polarization of said light source beam to producegenerally circularly polarized light in said signal beam.
 5. Theinterferometer of claim 4 wherein said means for changing said polarizedstate is a quarter wave or lesser fraction plate.
 6. An optical homodynepolarizing interferometer comprising:a) a source of light providing alight source beam; b) means for splitting said light source beam into asignal light beam and a reference light beam; c) means for forming asignal beam path along which said signal beam travels to and from anobject whose surface is to be measured and means to form a referencebeam path along which said reference beam travels, said reference beampath and said signal beam path being approximately optically equal inlength and within a coherence length of said light source beam; d) meansfor linearly polarizing in a vertical plane one of said signal beam andsaid reference beam and linearly polarizing the other one of said signalbeam and said reference beam in a horizontal plane; e) means to combinesaid linearly polarized reference beam and said linearly polarizedsignal beam into a resultant beam; f) means to split said resultant beaminto two equal light projections; g) means to split one of said lightprojections into two orthogonal, linearly polarized light projectionsthereby forming first and second orthogonal linearly polarized lightprojections; h) means to circularly polarize the other one of said lightprojections and split said circularly polarized light projections intotwo orthogonal, linearly polarized light projections thereby formingthird and fourth orthogonal, linearly polarized light projections; i)photon detecting means for generating electrical signals for each ofsaid first, second, third and fourth orthogonal, linearly polarizedlight projection correlated to the intensity thereof; and j) means forconditioning said signals to measure a path difference between saidsignal beam and said reference beam from one instantaneous readingthereof.
 7. An optical homodyne polarizing interferometer as set forthin claim 6 wherein there are four photon detecting means individual onesof which output a signal that is correlated to an intensity of one ofsaid first, second, third and fourth orthogonal, linearly polarizedlight projections, and wherein said conditioning means is comprisedof:means for generating a first difference signal representing adifference between the signals corresponding to said first and secondorthogonal, linearly polarized light projections and for generating asecond difference signal representing a difference between the signalscorresponding to said third and fourth orthogonal, linearly polarizedlight projections; means for filtering each of said first and seconddifference signals for suppressing low frequency components thereof,said filtering means generating first and second filtered differencesignals; means for squaring said first filtered difference signal andfor squaring said second filtered difference signal; and means forsumming said first squared filtered difference signal and said secondsquared filtered difference signal to generate an output signal.
 8. Anequal path length homodyne polarizing interferometer comprising;a sourceof light generating a light source beam which is linearly polarized;means for splitting said light source beam into a signal light beam anda reference light beam; means for forming a signal beam path along whichsaid signal beam travels to and from an object of interest and means toform a reference beam path along which said reference beam travels, saidreference beam path and said signal beam path being approximatelyoptically equal in length and within a coherence length of said lightsource beam, said reference beam path including means for ellipticallypolarizing said reference beam, said signal beam path including meansfor circularly polarizing said signal beam; means to combine saidreference beam and said signal beam into a circularly polarizedresultant beam; means to split said resultant beam into two orthogonal,polarized light projections; a plurality of photon detecting means forsensing the intensity of each orthogonal projection and generatingelectrical signals correlated thereto; means for conditioning saidelectrical signals to measure a path difference between said signal beamand said reference beam; and optical means, interposed within saidreference beam path, for inducing a polarization in said reference beamto compensate for optical distortions; wherein said conditioning meansincludes means for squaring, summing and averaging said electricalsignals to improve the signal to noise ratio thereof.
 9. An equal pathlength homodyne polarizing interferometer comprising;a source of lightgenerating a light source beam; means for linearly polarizing saidsource light beam; means for splitting said light source beam into alinearly polarized signal light beam and a linearly polarized referencelight beam; means for forming a signal beam path along which said signalbeam travels to and from an object of interest and means to form areference beam path along which said reference beam travels, saidreference beam path and said signal beam path being approximatelyoptically equal in length and within a coherence length of said lightsource beam; means, interposed within said light source beam, forcontrolling a plane of polarization of said signal beam with respect toa plane of polarization of said reference beam; means to combine saidreference beam and said signal beam into a resultant beam; at least onephoton detecting means for detecting a phase difference between saidsignal beam and said reference beam and for generating an electricalsignal correlated to said phase difference; and means for conditioningsaid electrical signal to measure a path difference between said signalbeam and said reference beam, said conditioning means including meansfor squaring, summing and averaging a plurality of said electricalsignals to generate an output signal.